1. Field of the Invention
The present invention relates to an image reading device that reads an original document and generates image data of the original document, an image forming apparatus including the image reading device, and a method of installing a reading unit in the image reading device.
2. Description of the Related Art
FIG. 1 is a schematic diagram of a typical image reading device. The image reading device reads an image of an original document 101 passing between a reference whiteboard 102 and a reading unit 103. An output level of the reading unit 103 depends on various factors such as density of the reference whiteboard 102, distance between the reading unit 103 and the reference whiteboard 102, and variation of spectral characteristics among parts in the reading unit 103.
When the reading unit 103 reads a color original document, red, green, and blue (RGB) values can be uniformly affected by the first factor and the second factor. As a result, an image with the same hue but a different brightness can be obtained. Moreover, even when the reading unit 103 reads the same color image, a different ratio of RGB values can be obtained because of an effect of the third factor. As a result, an image with a different hue and a different brightness can be obtained.
If coefficients for red, green, and blue are set individually so that target RGB values can be obtained when a reference original document is read, the fist factor and the second factor cannot affect the output level. If color correction is performed based on a color-correction parameter that is defined from the RGB values obtained by reading a plurality of color original documents, the third factor cannot affect the output level.
There have been known technologies about the color correction disclosed in, for example, Japanese Patent Application Laid-open No. 2004-104239, Japanese Patent Application Laid-open No. 2000-059637, and Japanese Patent Application Laid-open No. 2000-216997. Japanese Patent Application Laid-open No. 2004-104239 discloses an image reading device that can optimizes a reading level so that a difference between a reading level (output level at which image data is output) using a sheet-through document feeder (SDF) and a reading level using a platen can be small. The image reading device includes a photoelectric converter, a reading optical unit, an analog-signal processing unit, and an analog-to-digital (AD) converter. The photoelectric converter converts image data optically obtained from the original document into an analog electric signal. The optical unit emits a light to the original document and leads the light reflected by the original document to the photoelectric converter. The analog-signal processing unit samples the analog signal generated by the photoelectric converter, and performs a gain adjustment. The AD converter converts the analog signal processed by the analog-signal processing unit into a digital signal. The image reading device has a first reading mode and a second reading mode. In the first reading mode, the reading optical unit reads the original document while the reading optical unit moves under the unmoving original document. In the second reading mode, the reading optical unit reads the original document while the original document moves above the unmoving reading optical unit. The image reading device includes a correction unit that corrects the output level so that the output level of the image data of the original document can be optimized in either the first reading mode or the second reading mode.
Japanese Patent Application Laid-open No. 2000-059637 discloses a method of obtaining constant color reproduction by correcting variation of spectral sensibilities in a scanner due to a light source, a color filter, and the like and reading errors due to the variation of spectral sensibilities. To obtain the constant color reproduction, spectral sensitivity characteristics of the image reading device are assumed based on a measurement value that is obtained by measuring a reference original document with a predetermined measuring unit or a characteristic value of the reference original document and a statistic value calculated by analyzing an image-signal value that is obtained by reading the reference original document with the image reading device. A color-correction parameter is defined based on the spectral sensitivity characteristics. An image signal obtained by the image reading device is corrected using the color-correction parameter.
Japanese Patent Application Laid-open No. 2000-216997 discloses an image reading method in which, if a plurality of types of image signals is obtained under different reading conditions, signal levels of the image signals can be matched with each other by performing correction based on a difference among predetermined reading light intensities not arising errors due to characteristics of the document reading unit but absorbing differences among the image signals (image density data) due to the different reading conditions. In the image reading method, the original document image is photoelectrically read under several types of reading conditions having different light intensities, and thereby several types of image signals are obtained. An image characteristic value is obtained from each of the image signals. The signal levels of the image signals are matched with each other by absorbing the differences among the signal levels using the image characteristic values.
In the typical reading device shown in FIG. 1, the output level can be free from the first to third factors, if required corrections are performed.
However, if, after a color-correction parameter of a color-correction circuit is set, a reading unit including the color-correction circuit is installed in the typical image reading device, a proper color correction cannot be performed. This is because the density of the reference whiteboard and the distance between the reading unit and the reference whiteboard at the installed state can differ from those when the color-correction parameter is set, and thereby an output level at an input side of the color-correction circuit can be changed.
Moreover, the conventional technologies disclosed in Japanese Patent Application Laid-open No. 2004-104239, Japanese Patent Application Laid-open No. 2000-059637, and Japanese Patent Application Laid-open No. 2000-216997 cannot correct the output level at the input side of the color-correction circuit.